OBS广角地震探测中二次反射Pg震相特征及在地壳结构成像中的作用

在海底地震仪(OBS,Ocean Bottom Seismometer)广角地震探测中,经常可以见到能量很强、连续性很好的多次波震相,但对于如何确定这些多次波的属性以及怎样充分利用它们来约束地下的结构特征,至今还缺乏详细的研究.本文利用南海北部已获取的OBS探测数据,对与地壳折射Pg震相近乎平行并紧随其后的二次反射Pg震相进行了系统分析研究,发现在实测OBS地震记录剖面上,二次反射Pg震相具有连续、清晰、可追踪偏移距较远(约60km以上)等特点,其地震波形和粒子运动轨迹与初至Pg震相相似,但二次反射Pg震相波形最大振幅值和粒子震动能量明显比初至Pg震相大.通过理论模型模拟以及对实测地震剖面三种不同反射层路径的走时计算,确定了二次反射Pg震相主要来自沉积层的反射.在此基础上,对二次反射Pg震相在地壳结构成像中的作用进行分析,发现加入二次反射Pg震相前后,由于反射震相的增加,沉积层界面的约束程度得到极大的提高;另外,通过对理论模型和实测剖面OBS2010地壳结构进行加入二次反射Pg震相前后的分辨率测试,结果发现加入二次反射Pg震相数据后,由于射线密度的增加,沉积层和上地壳结构的成像分辨率有显著改善.

Characteristics of secondary Pg phases from OBS wide-angle seismic survey and their role in crustal imaging

How to eliminate or utilize secondary waves is one of the significant topics in processing of seismic data of seismic exploration. Little work is concerned with this aspect of wide-angle seismic survey using the ocean bottom seismometer (OBS), especially taking advantage of the secondary waves to improve seismic imaging. This study attempts to deepen understanding of the characteristics of the secondary Pg phase and to discuss its application to seismic imaging in OBS wide-angle seismic survey.#br#We firstly identify the secondary Pg phase from synthetic seismogram sections and record waveforms, then calculate and analyze particle motions of primary Pg and secondary Pg phases through the azimuth angle rotation. With this knowledge, we determine the propagation path by the theoretical model simulation and calculation of measured data from the P wave travel forward modeling method based on the RAYINVR. Furthermore, we use the theoretical model and the actual model of OBS2010 to show the role of the crustal structure imaging.#br#The secondary Pg phase is roughly parallel to and follows closely the primary Pg phase, and is continuous and clear with strong amplitude. An obvious vibration is observed behind the vibration of the primary Pg with stronger amplitude, which is supposed to be generated by the secondary Pg phase. On the basis of particle motions, the secondary Pg phase belongs to the P-wave seismic phase. The travel-time fits of the possible propagation path based on the test data give three different results:(1) The ^χ2 value is 14.921 when the reflective layer is water and sediment. (2) The ^χ2 value is 193.264 when the reflective layer is only water. And (3) the ^χ2 value is 1.786 when the reflective layer is merely sediment.#br#Theoretical investigation and data tests lead to following conclusions:(1) The secondary Pg phase is actually a P-wave. (2) The secondary Pg phase is mainly from the reflection between the sediments. (3) This wave greatly increases the constraint to the basement. And (4) it can be used to improve the imaging resolution of the sediments and the upper crust.